trust-declarations-switch

As with all compiler switch variables, the value of this variable
can be t, nil, or a
function object that accepts four arguments and returns t or nil. The arguments
passed to the function will be the values of the safety, space, speed,
and debug optimization qualities, in that order. nil is equivalent to a function that always returns
nil and t to a
function that always returns t. When the
value is a function and we say t (or true) or
nil (or false) in the text below, we mean
that the function returns, respectively, t or
nil.

If true, the compiler will trust declarations in code (perhaps other than
dynamic-extent declarations -- see trust-dynamic-extent-declarations-switch)
and produce code (when it can) that is optimized given the
declarations. These declarations typically specify the type of values
of variables. If nil, declarations
will be ignored -- except (declare notinline) and (declare
special) which are always complied with.